Electrophotographic printers (i.e. laser printers) operate by exposing a charged photoconductive surface, typically the surface of a drum or plate, to laser light. In places that are contacted by the laser light. The electrostatic charge dissipates and certain types of toner particles adhere. These particles are then transferred to the surface of a piece of media, such as paper, plastic, and the like. Many printers may expose dots as small as 1/1,200 of a square inch to laser light. However, isolated dots of these small sizes cannot be reliably printed because the toner may not reliably adhere. Therefore, the resulting images may be objectionably “noisy” in their appearance.
To alleviate this problem, dots are typically printed in clusters having a larger cumulative size such as, for example, 1/150 of a square inch. These clusters can also be referred to as macro-dots. Although the laser printer toner may not be able to reliably adhere at 1/1,200 of a square inch resolution, the device can address the positioning of a cluster of dots to a resolution of 1/1,200 of a square inch and allows for high addressability and precision in placement of the clusters it produces, so the technology of these machines provides image quality advantages.
Since production of smaller dots will not likely increase the definition of the image due to the above problems, an alternative to working toward controlling the application of fine dots, such as those approximately 1/1,200 of a square inch, is to control the output of the laser during the formation of a cluster. This is accomplished by turning the laser on and off during a period when the cluster is formed on the photoconductive surface. In this way, the size of the macro dots, or clusters, can be made slightly larger or smaller.
Many laser and inkjet printers currently produced cannot print the many shades of gray or color (typically 256 or more shades) that are necessary to simulate continuous grayscale or multi-color images, also referred to as contone images. These printers typically only print utilizing one color ink, e.g. cyan, magenta, yellow or black ink and, therefore, at a given space on a page, the printer can either leave the space blank or place a dot of ink thereon. By changing the density of dots, or the size of the dots, on areas of the page, the simulation of contone images can be made. This process of mimicking a contone image through use of varied densities of dots is referred to as halftoning. In halftoning, the image being created is defined into a plurality of small cells. A number of dots are then arranged in a pattern within the cell. The number and the pattern of the dots are dependent upon the particular shade of gray or color that is to be simulated and upon the type of halftoning that is being utilized.
Originally, a halftoning procedure was performed by means of a screen, i.e. “screening.” With today's increase in the power of computers, halftoning is more and more frequently performed in a digital fashion by raster image processors (RIPs). The halftoning operation is a computationally intensive application, and the resulting image sizes are large, since in forming the image a pattern of dots representing a shade of gray must be mapped rather than, for example, just a reference to the particular shade of gray that is required in a portion of a grayscale image.
In order to simulate variable-sized halftone dots in computer printers, dithering is used, which creates clusters of dots in a halftone cell. The more dots printed in the cell, the darker the shade of gray that is depicted. As the screen frequency gets higher (i.e. more cells per inch), there is less room for dots in the cell, reducing the number of shades of gray or color that can be generated.
In low resolution printers, there is always a compromise between printer resolution (dots per inch or dpi) and screen frequency (lines per inch or lpi), which is the number of rows of halftone cells per inch. For example, in a 300 dpi printer, the 8×8 halftone cell required to create 64 shades of gray results in a very coarse 38 lines per inch of screen frequency (300 dpi divided by 8). However, a high resolution, 2,400 dpi printer can easily provide the appearance of 256 shades of gray at 150 lpi (2,400/16). At this resolution, the human viewer cannot distinguish black and white dots from continuous gray.
Halftone images, also called bi-level or multi-level images, tend to be very large. That is, the image can range from a few megabits (Mbit) up to several gigabits (Gbit). When images are screened at a high resolution, the size of the halftone image can be several times larger than the size of the original contone image. Hence, storage and transmission of these images can benefit from compression.
However, halftones are not typically compressible, and the methods that have been developed to date often resort to a loss of image information during the compression process called “lossy compression”. Previous methods to compress halftone images may act to convolve the images with low-pass filters to convert them back to contone images which are then compressed with such well-known techniques such as JPEG, but this technique is a lossy compression method and therefore results in a loss of image information and quality.
High-resolution digital printers and growing computational requirements associated with new applications, such as printing-on-demand and personalized printing have increased the need for fast and efficient lossless halftone image compression.